Newcastle signed Japan forward Yoshinori Muto from Mainz on a four-year contract on Thursday, hours after manager Rafa Benitez hinted at concerns over a lack of transfer spending following a 4-0 friendly defeat.

Newcastle said last week that they had reached an agreement with the Bundesliga club for the transfer. Muto's move was completed on Thursday after he was granted a work permit.

Muto, 26, has played 25 times for Japan and featured in one game at the recent World Cup, the 1-0 loss to Poland. Reports said Muto's transfer fee was around £9.5 million.

"I'm honoured to be a player for such a big club," said Muto, the first Japanese player to join Newcastle.

We are delighted to announce that Japan international Yoshinori Muto has completed a move to Newcastle United after he was granted a work permit at a hearing today. #NUFCpic.twitter.com/eB7qAoxtsa

He is Newcastle's fourth permanent signing of the close season following goalkeeper Martin Dubravka, midfielder Ki Sung-Yueng and defender Fabian Schar, with the club moving to add another option to its forward line after selling Aleksandar Mitrovic to Fulham this week.

A day earlier, Benitez expressed displeasure with his club's summer activity when was not pleased with the Magpies' performance in a thrashing at Portuguese club Braga.

"Things are not going well off the pitch and you can see a reflection of that on the pitch," Benitez said on Wednesday, adding that he wanted to sign three of four players before next week's transfer deadline.

"You see the teams that have been promoted, the money they are spending. Put it all together and you understand why the fans need to be concerned.

"We are concerned. Everyone in the dressing room was really upset with our performance and with how things are going, but we will try to change things in the next 10 days."

Newcastle begin their league campaign by hosting Tottenham at St James' Park on Aug. 11.

Information from The Associated Press and Reuters was used in this report.